Telephone networks comprise a series of interconnected subsystems that are linked together at points called interfaces. These interfaces provide a link between old and new equipment and allow for simplified design and maintenance. A local loop is an example of an interface that connects a subscriber's telephone set and a central office.
The isolation between the line side (the side having a direct connection to the central office) and the computer side (the side having a direct connection to the user premises equipment) is often accomplished within the interface circuit. A modem is an example of an interface circuit that may include circuitry that provides electrical isolation from the line in addition to the signal modulation and demodulation function of the modem. Isolation transformers, optical coupling, and capacitive coupling are all examples of known methods of isolating the line side from the computer side.
FIG. 1 is a block diagram of a typical telecommunication system 5 showing the connection between a subscriber and a central office that controls the telecommunication system. Central-office equipment 10 on the line side 15 of the telecommunication system 5 is connected to user device 20 (e.g., telephones or computer terminals) on the computer side 25 of the telephone system 5 via an interface circuit 30.
The DC power inherent in a telephone line provides a convenient source of power, but there are often limitations and restrictions which limit the ability of a modem to derive power from the telephone line. For instance, present regulations in the United States require that significant current may only be drawn from the telephone line when the telephone or modem is in an off-hook or active condition.
The U.S. Federal Communication Commission (FCC) and other counterpart regulatory agencies in other countries also require electrical isolation between the line side and the user devices on the computer side. Electrical isolation protects the line side from damage transmitted from the computer side and vice-versa. Many components (e.g., data access arrangements (DAAs) or CODEC's) of telephone interface circuits are PSTN line-powered circuits, i.e. they operate from PSTN line current because they are isolated from the low voltage power supplies. The DAA must provide isolation between the low-voltage computer side and the high-voltage line side. Because the line-powered interface circuits are isolated from low voltage power supplies and the amount of line current available to operate the interface circuits is severely limited during the on hook state by PSTN regulations, the on-hook functions are difficult to perform without the use of expensive transformers and/or optocouplers.
When a circuit on the line side is placed in the “on-hook” state (e.g., a telephone receiver on the computer side is placed in its cradle) the local loop is opened and almost all of the power to the interface circuit is cut off. Activating the DAA or CODEC requires that some power be drawn from the local loop or from another source.
While in the on-hook state a small amount of current (idle-state loop current or leakage current) can be drawn for a short period of time from the TIP/RING line to operate the DAA or CODEC. Typically, the maximum AC loop current that can be drawn from the TIP/RING line is about 500 μA DC during ringing and about 200 μA during caller ID transmission. The maximum amount of leakage current allowed is only 7 μA of DC current. Accordingly, complicated power management techniques have been implemented which must be used so that the power for the DAA or CODEC is minimized and is allocated only when it is clear that the power is necessary. For example, systems have been developed that will operate in a low power mode (e.g., 20 μA ) during a discrimination stage where a determination is made that a real event (as opposed to line noise which appears to be an event) has occurred, and then switch to a medium power mode (e.g., 50 μA ) during a transmission mode where the event data is transmitted across the capacitive interface. Since there is so little current available during the on-hook state, it is extremely difficult to power the required circuits needed to perform on-hook functions.
In view of the tight limitations relating to the drawing of power, it would be desirable to develop a power source that would not derive power from leakage current, idle-state loop current and that would instead utilize otherwise “discarded” power to isolated circuits.